The present invention relates to the field of flexible tubular pipes for transporting hydrocarbons, especially unbonded flexible pipes. These pipes are defined in the recommendations API 17J and 17B of the American Petroleum Institute and comprise metal layers and separate polymeric layers, that is to say layers that are not bonded together so as to allow certain relative displacement between the layers.
More precisely, an unbonded pipe of the type intended in the invention generally comprises, from the inside to the outside:
an internal sealing sheath made of a plastic, generally a polymer, resistant to the chemical action of the fluid to be transported;
optionally, a pressure vault resistant mainly to the pressure developed by the fluid in the sealing sheath and consisting of the winding of one or more interlocked metal profiled wires (which may or may not be self-interlockable) that are wound in a helix with a short pitch (i.e. with a winding angle close to 90° with respect to the axis of the pipe); the profiled wires have a cross section in the form of a Z or a T, or derivatives (theta or zeta) thereof, or a U or an I;
at least one ply (and generally at least two crossed plies) of tensile armor wires wound with a long pitch—the lay angle measured along the longitudinal axis of the pipe is, for example, approximately equal to 55°; and
optionally, an external protective sealing sheath made of a polymer.
Such a pipe may be what is called a “smooth bore” pipe when the bore is formed directly by the sealing sheath or what is called a “rough bore” pipe when a carcass consisting of an interlocked metal strip wound in a short pitch is also provided inside the internal sealing sheath, said carcass serving to prevent the pipe from collapsing under the external pressure. When a carcass is used, it is possible for certain applications to dispense with the pressure vault.
Optionally, the pipe may include, in addition to these layers, other special layers, a metal hoop (wound in a short pitch) and forming part of the pressure vault, intermediate polymeric sheaths, etc.
The precise construction, number and arrangement of the various layers are carefully chosen depending on the applications and the operating conditions of the pipe, but in all the pipes there are layers formed by windings of steel reinforcing or armor wires.
For deep-sea applications, which are the intended main applications of the invention, the pipe generally comprises all of the following: a carcass, a sealing sheath, a pressure vault, tensile armor plies and an external sealing sheath.
Within the meaning of the present invention, the armor wires in question are the tensile armor wires of the crossed armor plies or else possibly the profiled wires or the hoop wires of the pressure vault, which will be called pressure armor wires. By extension, armor wire will also be understood to mean a profiled wire which is obtained by the process of the invention and would be intended to be used for manufacturing a carcass.
When the pipes are intended to operate in an acid corrosive medium (especially because of the H2S contained in the effluents transported), which is often called in the oil industry jargon a “sour” medium, it is necessary to adopt special measures in order to guarantee the corrosion resistance of the armor (tensile and pressure) wires. These measures and the grades of steels that are necessary are defined in the NACE (National Association of Corrosion Engineers) standard MR01-75 governing the corrosion resistance in sour medium of steels and alloys.
Usually, steels with good H2S corrosion resistance have relatively poor mechanical properties (Rm<850 MPa). Now, if the envisioned operating conditions are both corrosive and deep sea, it is necessary to preserve the mechanical properties of the metal wires, especially the armor wires, which will be subjected both to corrosion and to the high tensile forces encountered (for a seabed flowline transport pipe, these high tensile forces not perhaps occurring during the life of the pipe once laid, but at least while the pipe is being laid). If the steel does not have very good mechanical properties, it is necessary to increase the steel thicknesses used, which increases the weight of the pipes, the size of the winding and laying equipment and therefore the manufacturing cost of the pipes.
According to document FR 2 775 050, which relates to an unbonded flexible pipe intended for static use in a corrosive environment, a steel resistant to H2S corrosion but with moderate mechanical properties is used for the armor wires of the pressure vault, while a steel having high mechanical properties but not resistant to sour corrosion is used for the tensile armor plies. This compromise appears acceptable if the H2S corrosion cannot reach the tensile armor plies; for this purpose, an intermediate H2S confinement sheath separates the pressure vault, which will undergo H2S corrosion, from the tensile armor plies, which in principle will not undergo this corrosion. However, safety is not guaranteed because of the risks of the intermediate sheath being pierced. Moreover, the poor mechanical properties of the steel used for the pressure vault mean that they have to be oversized.
In the field of bonded pipes, the document FR 2 569 461 discloses a rubber hose intended for transporting corrosive effluents and incorporating, for this purpose, reinforcements consisting of layers of embedded metal cables, the cables consisting of steel wires coated with plated aluminum (that is to say with intimate bonding to the steel, obtained under high application pressure, for example by coextrusion). This hose, manufactured using the technology of bonded pipes, therefore has a construction different from that of the unbonded pipes mainly envisioned according to the invention, which are subjected to tensile stresses that cannot be envisioned with this bonded pipe technology. If the aim is to retain the beneficial concept of using plated armors in pipes of the type more particularly considered in the present invention, it is necessary to envision plating the metal wire, which has high mechanical properties (Rm greater than 1000 MPa and preferably greater than 1400 MPa), with a corrosion-resistant coating. However, the use of such high-mechanical-performance metal wires coated with an anticorrosion plating is not entirely satisfactory, especially because of a difficulty arising from the fact that the intimate bond between the coating and the steel is brittle and cannot withstand the stresses associated with the actual pipe manufacture.